fun thing is that the last piece of heart in
MM was the one the beavers give to you when you beat them for the 2nd time, weird because I love that mini game, can't believe i never tried it before... anyway this was a month ago, I have already everything 100% on
MM.
Retrospectively, the most hard to get in
MM would be a tie between the poe fight on Ikana, near the musical box house, took me a few times. Also the shooting gallery of clock town, took me a lot of tried until i memorized everything. and i remember when i was first trying the deku game on north clock town i thought it was hard, but then i tried again doing the three days, and it wasn't hard at all... but it wasn't easy either, so i think those 3 are the hardest ones, i think the shooting gallery is the hardest though.
I'd also say the ones where you need the scarecrow, I had to use guides on those, but once you know it's not hard at all...